Using X-ray fluorescence analysis to detect and measure the presence of precious metals like gold and/or platinum group metals in a sample requires the excitation radiation to be energetic enough, because said metals have their characteristic X-ray fluorescence peaks relatively far on the energy axis. The same is actually true for all sample constituents that have high enough X-ray fluorescence energies, but precious metals have particular significance because X-ray fluorescence is an important analytical technique to be used for sorting ore in a large industrial scale.
An X-ray tube is traditionally used in an X-ray fluorescence analyser to produce the desired excitation radiation. Electrons emitted from a cathode are accelerated and made to hit an anode. When the accelerated electrons interact with the atoms of the anode, high-intensity X-ray radiation is produced at characteristic energies of the anode material. Additionally there comes the so-called bremsstrahlung, which is X-ray radiation at a continuous distribution of energies. The highest-energy bremsstrahlung comesat energies higher than the characteristic peaks, with the bremsstrahlung cut-off energy corresponding to the voltage used to accelerate the electrons (for example an X-ray tube using a 60 kV acceleration voltage can produce bremsstrahlung photons not more energetic than 60 keV). In order to analyse heavy elements like the precious metals mentioned above through X-ray fluorescence, the most energetic part of the bremstrahlung is typically needed for excitation. Few or no anode materials are known that would have high enough characteristic energies that could be used as excitation radiation and that would avoid overlapping with the characteristic energies of the elements to be measured.
In an X-ray fluorescence analysis where highly energetic excitation radiation is needed, the lower end of the bremsstrahlung (and, actually, even the characteristic peaks of the anode material) is only a nuisance. Photons of the excitation radiation get scattered by the sample material, and a quite significant number of them find their way into the detector, causing continuous-spectrum background noise. Especially the signal processing electronics coupled to an energy dispersive detector are unnecessarily loaded by scattered excitation radiation that does not carry any meaningful information of the sample material.
A prior art XRF analyser is schematically illustrated in FIG. 1. The illustrated analyser configuration uses an X-ray tube of the so-called side window type. Electrons become detached from a cathode 101 and are accelerated towards an anode 102 with a high voltage coupled between the electrodes. As a result, a beam of X-rays 103 is generated that exits the X-ray tube through a window 104 on its side. A heating unit 105 is needed to heat up the cathode 101, and a cooling unit 106 transfers away the heat generated in the anode 102 by that part of the accelerated electrons' energy that did not exit the X-ray tube in the form of X-rays. A filter 107, conventionally referred to as the primary filter, is placed on the path of the beam of X-rays 103, in order to shape its energy spectrum. The filtered beam of excitation radiation 108 hits the sample 109, the element composition of which is to be analysed. As a result, fluorescent radiation 110 is produced. A detector 111 receives some of the fluorescent radiation 110 and produces a measurement signal, which is processed further in processing electronics 112.
Assuming that the analyser is built for analysing heavy elements in the sample 109, the main purpose of the primary filter 107 is to absorb that part of the generated beam of X-rays 103 that is too soft to be used as excitation radiation, or more specifically those of the generated X-rays that would overlap with the characteristic peaks of the sample material(s) to be analysed. FIG. 2 illustrates a schematic comparison of the originally generated beam of X-rays 103 and the filtered beam of excitation radiation 108. Most importantly the long “tail” that in the upper diagram represents the soft end of the bremsstrahlung is missing in the lower diagram. However, also the overall intensity of the highest-energy X-rays is decreased. This comparison underlines an important drawback of prior art solutions: simply filtering out the softer X-rays will inevitably also affect the intensity of the harder, desired X-rays. In order to produce a high enough intensity of excitation radiation in the arrangement of FIG. 1, the X-ray tube must be operated at a very high power, which means e.g. using relatively large amounts of energy in both the heating unit 105 and the cooling unit 106.